BUG 1: Sidebar buttons clicked wrong plugin
- Lambda captured idx by reference, always using last value
- Fixed by using functools.partial to capture by value
BUG 2: Dashboard plugin KeyError 'border_subtle'
- Changed to 'border_default' which exists in EU_COLORS
The sidebar should now correctly switch to the clicked plugin.
BUG: 'bg_panel' KeyError in Dashboard plugin.
The EU_COLORS dictionary doesn't have 'bg_panel'. Changed to use
'bg_secondary' which is the correct background color for panels.